Welded stainless steel pipes mostly adopt stainless steel containing no magnetic by argon arc welding and high frequency induction welding and forming, the stainless steel material to chromium (Cr), nickel (Ni) as the main alloy elements, and a small amount of carbon (C), titanium (Ti) and other elements. Generally this kind of stainless steel containing Cr18%, Ni content > 8%, under normal circumstances, this kind of steel before welding organization for single austenite, so it has good corrosion resistance, but not including magnetic. But this kind of stainless steel strip by molding, welding basically are both sides of weld seam uneven hardness, corrosion resistance is poor shortcomings.
In fact, any kind of metal materials are all from the grain of a stacked, in the plane, each grain can be regarded as an irregular polygon (mostly with hexagon), adjacent to the grain boundary is the common. Usually, austenitic stainless steel single the grain boundary composition and grain composition is the same, namely the alloy content everywhere especially average composition content of Cr are close to the material, but when welding, because of high temperature of welding seam, end, rapid cooling austenite still get uniform, so is the weld is keep good ductility and corrosion resistance; but in the weld (heating temperature is 400 ℃ ~ 850 ℃ this range), due to the welding process and the heating temperature is low, the formation of Cr23C6 alloy carbides at the grain boundaries of Cr and C, Cr content in the matrix grain boundary is greatly reduced in the. As everyone knows, Cr content and corrosion resistance of steel matrix, the general content of Cr is higher, its corrosion resistance is strong.
The content of Cr at grain boundaries of much lower, so its corrosion resistance is greatly reduced, thus the welded steel pipe corrosion resistance is greatly reduced. In addition, in both sides of weld seam, due to temperature is appropriate, alloy carbides in the matrix (such as TiC) can migrate to the grain boundaries, thus the hardness is increased. This is also the weld decrease the corrosion resistance and hardness is the main reason for the increase of.
In order to ensure its performance, especially the corrosion resistance performance, under normal circumstances, stainless steel pipe welding should be conducted after the heat treatment, the heat treatment is solid solution treatment (also known as the annealing treatment, the title some discretion).
Edit by Mr. Bruce Zhang of KRM/BSW
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